The liver is the most important organ in the body for clearance of bacteria from the bloodstream, and prevention of septicemia and sepsis; the mechanisms are unclear. The majority of studies undertaken to date have focused on the function of resident tissue macrophages (Kupffer cells) that line the liver sinusoids, ndeed, it is often suggested that Kupffer cells ingest and kill the bulk of organisms taken up. Experiments reported first in our laboratory indicate that the actual mechanisms are far more complex. Rather, bacterial elimination is dependent upon the interaction of Kupffer cells and bactericidal neutrophils that immigrate rapidly in response to infection. The factors that effect neutrophil removal and resolution of the inflammatory response have not been addressed. Studies demonstrating neutrophils inside the Kupffer cells of infected mice suggest that neutrophil-Kupffer cell interaction may be crucial. It is hypothesize that this interaction is essential to inhibiting the uncontrolled discharge of toxic metabolic products by neutrophils, suppressing the pro-inflammatory activity of Kupffer cells, and preventing chronic liver inflammation. The SPECIFIC AIMS of this proposal are to: I. Define the role of Kupffer cells in regulating the inflammatory response of neutrophils to bacteria cleared by the liver; II. Determine the effect of neutrophils on cytokine production by Kupffer cells; and III.Contrast the factors that affect neutrophil-Kupffer cell interaction with the effects of those same factors on the interaction of neutrophils with mononuclear phagocytes that infiltrate the liver subsequent to infection. To avoid the artifacts that can plague other methodologies, experimentation will rely largely on quantitative real-time RT-analysisof the genes expressed by cells obtained by Laser Capture Microdissection. Experiments that examine the activities of purified Kupffer cells and neutrophil in vitro will corroborate these analyses. The results should drastically alter our current understanding of the roles of Kupffer cells, neutrophils, and the factors that regulate the inflammatory response to bacteria that invade the bloodstream. These, in turn, will afford a new understanding of the maladaptive responses to systemic infections, as well as to other pathological events that culminate in liver injury and organ failure. Consequently, they should provide valuable insights that enable the development of innovative strategies to improve treatment and prevent the devastating consequencesoften associated with septicemia and sepsis.